The manufacture of polyurethane flexible foams by reacting organic polyisocyanates such as tolylene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) with polyether polyols in conjunction with a foaming agent is well established. The polyethers are usually polyoxypropylene polyols derived from propylene oxide or poly(oxypropylene-oxyethylene) polyols derived from various combinations of propylene and ethylene oxides. Ethylene oxide tipped polyoxypropylene polyols wherein the oxyethylene groups constitute a minor proportion of the total oxyalkylene residues are particularly important because of their enhanced reactivity towards isocyanates.
Polyols having higher oxyethylene contents, for example 50% or more on a weight basis, are often employed as minor additives to ensure that the foams have an open-cell structure. The use of these polyethers at very high concentrations in conjunction with the usual isocyanates is not possible because then, instead of having a cell-opening effect, they result in closed cell foam.
It has now been found that flexible foam having valuable properties can be successfully made from formulations containing high concentrations of polyols having high oxyethylene contents if substantially pure 4,4xe2x80x2-MDI or a derivative thereof is employed as the polyisocyanate.
Thus according to the invention , there is provided a method for the preparation of flexible polyurethane foams by reacting in the presence of a foaming agent a polyisocyanate containing at least 85%, preferably at least 90% and more preferably at least 95% by weight of 4,4xe2x80x2-diphenylmethane diisocyanate or a variant thereof with a polyol composition comprising at least one poly(oxyethylene-oxyalkylene) polyol having an average nominal hydroxyl functionality of from 2 to 6, an average hydroxyl equivalent weight of from 1000 to 3000 and an average oxyethylene content of from 50 to 85, preferably 60 to 85% by weight, preferably the oxyethylene content of any further optionally present polyol having an equivalent weight of 500 or more being at least 50% by weight.
The polyisocyanate component used in the method of the invention may consist essentially of pure 4,4xe2x80x2-diphenylmethane diisocyanate or mixtures of that diisocyanate with one or more other organic polyisocyanates, especially other diphenylmethane diisocyanate isomers, for example the 2,4xe2x80x2-isomer optionally in conjunction with the 2,2xe2x80x2-isomer. The polyisocyanate component may also be an MDI variant derived from a polyisocyanate composition containing at least 85% by weight of 4,4xe2x80x2-diphenylmethane diisocyanate. MDI variants are well known in the art and, for use in accordance with the invention, particularly include liquid products obtained by introducing uretonimine and/or carbodiimide groups into said polyisocyanate composition and/or by reacting such a composition with one or more polyols.
The polyol composition may comprise a single polyoxyalkylene polyol, preferably a poly(oxyethylene-oxypropylene) polyol, having the required functionality, equivalent weight and oxyethylene content. Such polyols are known in the art and may be obtained in conventional manner by reacting ethylene and propylene oxides simultaneously and/or sequentially in any order with an initiator such as water, a polyol, a hydroxylamine, a polyamine and the like having from 2 to 6 active hydrogen atoms.
Alternatively, the polyol composition may comprise a mixture of two or more polyoxyalkylene polyols such that the total composition has the required average functionality, equivalent weight and oxyethylene content. The polyoxyalkylene polyols present in such mixtures are preferably poly(oxyethylene-oxypropylene) polyols but one or more polyoxyethylene polyols and/or polyoxypropylene polyols may also be present.
Preferred polyol compositions comprise :
(a) from 85 to 100% by weight of a polyoxyalkylene polyol containing oxyethylene residues, said polyol component having an average nominal hydroxyl functionality of from 2 to 6, an average hydroxyl equivalent weight of from 1000 to 3000 and an average oxyethylene content of from 50 to 85% by weight, and
(b) from 15 to 0% by weight of one or more other polyols.
Polyol (b) suitably has an average functionality of from 2 to 6, an average equivalent weight of 1000 to 3000 and may be, for example, a polyoxypropylene polyol, a polyoxyethylene polyol or a poly(oxyethylene-oxypropylene) polyol containing less than 50% or more than 85% by weight of oxyethylene residues.
Most preferably each polyol having an equivalent weight of 500 or more has an oxyethylene content of at least 50% by weight. The polyoxyalkylene polyols present in such mixtures are preferably poly(oxyethylene-oxypropylene) polyols but one or more polyoxyethylene polyols may also be present.
The term xe2x80x9caverage nominal hydroxyl functionalityxe2x80x9d is used herein to indicate the average functionality (number of hydroxyl groups per molecule) of the polyol composition on the assumption that the average functionality of the polyoxyalkylene polyols present therein is identical with the average functionality (number of active hydrogen atoms per molecule) of the initiator(s) used in their preparation although in practice it will often be somewhat less because of some terminal unsaturation. It is preferred that the average nominal hydroxyl functionality of the polyol composition is from 2 to 4, the most preferred polyoxyalkylene polyols being triols.
If desired, the polyoxyalkylene polyol (or one or more of the polyoxyalkylene polyols when a mixture of such is used) may contain dispersed polymer particles. Such polymer-modified polyols have been fully described in the prior art and include products obtained by the in situ polymerisation of one or more vinyl monomers, for example acrylonitrile and styrene, in polyoxyalkylene polyols or by the in situ reaction between a polyisocyanate and an amino- or hydroxy-functional compound, for example triethanolamine, in the polyoxyalkylene polyol.
The preferred foaming agent for use in the method of the invention is water, optionally in conjunction with a physical blowing agent, for example a low boiling organo fluoro compound. The amount of foaming agent may be varied in known manner in order to achieve the desired foam density, suitable amounts of water being, for example, from 0.25 to 20% by weight based on the weight of polyol composition. Preferably water is the only foaming agent. The isocyanate index of the reaction system, taking account of the polyol composition, water and any other isocyanate-reactive species, for example chain extenders or cross-linking agents, may be as low as 10 or as high as 120.
The foam-forming reaction mixture may contain one or more of the additives conventional to such reaction mixtures. Such additives include catalysts, for example tertiary amines and tin compounds, surface-active agents and foam stabilisers, for example siloxane-oxyalkylene copolymers, chain extenders, for example low molecular weight diols or diamines, cross-linking agents, for example triethanolamine, flame retardants, organic and inorganic fillers, pigments,agents for suppressing the so-called boiling-foam effect like poly-dimethylsiloxanes and internal mould release agents.
Accordingly, in a further aspect, the invention provides a reaction system comprising:
(i) a polyisocyanate containing at least 85% by weight of 4,4xe2x80x2-diphenylmethane diisocyanate or a variant thereof;
(ii) a polyol composition comprising at least one polyoxyalkylene polyol containing oxyethylene residues, said polyol composition having an average nominal hydroxyl functionality of from 2 to 6, an average hydroxyl equivalent weight of from 1000 to 3000 and an average oxyethylene content of from 50 to 85% by weight, preferably the oxyethylene content of each polyol having an equivalent weight of 500 or more present in the polyol composition being at least 50% by weight;
(iii)a foaming agent comprising water, and, optionally,
(iv) one or more additives conventional to foam formulations.
This reaction system is used for making flexible foams.
The term xe2x80x9creaction systemxe2x80x9d is defined as a system wherein the polyisocyanate is kept in a container separate from the isocyanate-reactive ingredients.
In operating the method of the invention, the known one-shot semi-prepolymer and full prepolymer techniques may be used together with conventional mixing equipment and the foams may be produced in the form of slabstock, mouldings and the like. Prepolymers having an NCO value of 5-30% by weight which are the reaction product of the above polyisocyanates (in excess) and polyols also form part of the invention. The preparation of such prepolymers is generally known in the art. Foams made by the method of the invention have valuable energy-absorbing properties making them useful for acoustic vibration damping as well as for seating purposes.